1. Field Of The Invention
The field of the present invention generally relates to high power solid state power supplies. More particularly, the field of the present invention relates to a high power solid state power supply for producing a controllable, constant high voltage output under varying and arcing loads suitable for powering an ion source, such as an electron beam gun in a vacuum furnace, or an electron beam gun used in the vaporizer of a laser isotope separation system, or a plasma sputtering device or the like.
2. The Prior Art
An electron beam gun is used in a vacuum furnace system, or the like, for providing a high intensity beam of electrons to bombard a target material. The electron gun is typically disposed in an evacuated chamber together with the target material. The electron gun or E-beam gun usually includes a source of electrons, such as a heated cathode or filament, and a grounded accelerating anode. The cathode is maintained at a high negative potential with respect to the anode to establish a high electrostatic field for accelerating the electrons. A magnetic field may typically be provided for directing the electrons onto the target material.
During bombardment of the target material by the electron beam, various ionized materials are emitted. The presence of such materials often effects a substantial decrease in the voltage withstand capability between the various parts of the electron beam gun and other elements. This may result in arcing between the electron gun parts and other structures. Arcing causes a substantial increase in the electron gun current and may result in damage to the electron gun structure and surrounding elements. Arcing may also cause damage to the power circuitry driving the electron gun.
In high power and high performance applications, such as the vaporizer in a laser isotope separation system, physical spacing between the E-beam gun, surrounding components, and target materials is relatively small. As a result, the E-beam gun may arc to ground frequently. To avoid damage and to achieve long lifetimes, it is essential that the energy stored in the power supply output capacitance be small and that the so called power supply let through energy during arcing be small. In addition, the close physical spacing causes a greater chance for the electron beam to impinge on adjacent components and structures during steady, non-arcing operation. To avoid this, it is important that the power supply output voltage be accurately controllable with low ripple content.
Conventional thyristor controlled power supplies are inadequate for high power and high performance electron beam gun applications. Thyristor controlled power supplies generally operate at 60 Hz line frequency and generate significant output voltage ripple or require substantial output capacitance to reduce the ripple to acceptable levels. If gun arcs are frequent, the output capacitance may result in excessive accumulated energy discharge into the gun or surrounding components and result in a short lifetime. Thyristor controlled power supplies also have a relatively slow dynamic response which results in further energy let through to the gun during the arc and slow ramp up after the arc is extinguished. Thyristor controlled power supplies have a relatively poor input power factor and generate high input harmonics. This causes substantial cost increases in the 60 Hz utility power system in large power applications. Thyristor controlled power supplies are also physically large because the transformer and filter components operate at 60 Hz and the lower harmonics of 60 Hz. This is an important factor in capital equipment costs where large numbers of power supplies are used.
Conventional power supplies utilizing series pass tetrode vacuum tubes eliminate many of the deficiencies of the thyristor controlled power supply. The regulating characteristics of the tetrode vacuum tube can be used to produce very low output ripple voltages without requiring significant output capacitance. The regulating characteristics also permit a diode rectifier front end to be used which greatly raises the input power factor and reduces the input line harmonics. The current limiting tube characteristics, the high speed control capability of the tetrode grid, and the low output capacitance provide excellent response to gun arcs resulting in low energy into the gun and fast recovery after the arc extinguishes.
However, conventional tetrode vacuum tube E-beam power supplies have serious deficiencies of their own. The efficiency of this type of power supply is 80% or less compared to approximately 95% for thyristor controlled power supplies. This is because the tetrode must drop substantial voltage continuously for it to regulate properly. Tetrode vacuum tubes also wear out due to the filament breaking and to the chemical breakdown of the coating on the cathode which causes the cathode to lose its ability to emit electrons. As a result, the tetrode vacuum tube is a substantial maintenance expense item having to be replaced at least every 10,000 hours.
Power supplies which use switch-mode dc--dc converters operating at 10 kHz and above have the potential to eliminate the deficiencies of the conventional thyristor controlled and series-pass tetrode type power supplies. Power supplies which employ switch-mode dc--dc converters are compact because of smaller transformer and filter components, operate with a diode rectifier input for high input power factor, are efficient because they do not operate as linear regulators, require low maintenance because they are all solid state, and can have good dynamic response because they operate at high frequency.
One type of switching dc--dc converter useful for high power applications above 10 KHz with arcing loads is the series resonant type. Power supplies which use series resonant type dc--dc converters have an input rectifier and filter to produce a dc voltage, an inverter consisting of thyristors and a resonant network to produce high frequency current, a transformer for producing the desired output voltage level, and a rectifier and filter to produce dc for application to the load. This is a well known type of power supply which has been applied to E-beam guns (U.S. Pat. No. 3,544,913, issued Dec. 1, 1970). The major deficiencies in this type of power supply for high performance E-beam applications are the amount of energy stored in the output filter capacitance and the inability to turn off power to the load until the resonant network reverses polarity. The output current of the inverter is sinusoidal and a substantial capacitance is required after rectification to obtain satisfactory output voltage ripple even though the inverter operates above 10 KHz. The dc--dc converter also continues to provide current to the load after an arc occurs until the resonant network commutates the thyristors. Although superior to the conventional 60 Hz thyristor type power supply with respect to energy dissipated into the gun during arcing, it is inferior to the series pass tetrode type power supply, and is not adequate for high performance power supplies for E-beam guns.
Another type of switching dc--dc converter useful for low power applications up to a few kilowatts and arcing loads is the current source, pulse-width-modulated type. This dc--dc converter consists of a voltage regulator, inductor, non-regulating inverter, transformer, output rectifier, and output filter capacitor as described in U.S. Pat. No. 3,737,755, issued Jun. 5, 1973. The inductor and inverter described in this referenced patent produce a square current waveform to the output rectifier and filter which allows a small output filter capacitance to be used and therefore low energy to the load during load arcs. However, the inverter voltage clamping means is inadequate for high power applications. This is because the inverter is relatively distant from the input filter capacitor in high power applications which results in substantial inductance in the clamping network and excessive voltage spikes across the inverter transistors.
As described above, problems exist with conventional power supplies and with switching power supplies for high power and high performance ion sources and specifically, electron beam guns. In summary, conventional thyristor power supplies have high output capacitance, slow dynamic response to arcing, and poor input power factor. Series pass tetrode regulator type power supplies have relatively low efficiency and substantial maintenance expense related to the vacuum tube. Both of these conventional types of power supplies are also physically large. Switching power supplies using series resonant type dc--dc converters solve many of the problems associated with conventional power supplies but still have excessive output capacitance and too slow a response to arcing. Switching power supplies using current source, pulse-width-modulated type dc--dc converters as described in the prior art potentially meet the E-beam gun power supply requirements but do not operate at high power levels.
Accordingly, it is an object of the present invention to provide an improved current source, pulse-width-modulated type dc--dc converter suitable for operation at 100 kW or more.
It is another object of the present invention to provide a power supply which is modular with one or more dc--dc converter modules of identical design rated at 100 kW or more used to achieve the required output power.
It is also an object of the present invention to provide a power supply which has tight voltage regulation and low output voltage ripple for precise beam control as well as small output capacitance for small energy into the load during load arcs.
A further object of the present invention is to provide a power supply which is current limited during an arc, which cuts back power to zero within a few micro-seconds or less after an arc is initiated, and which then ramps power back on in several milli-seconds after the cutback interval.
A still further object of the present invention is to provide a power supply which operates without excessive voltage transients or cable reflections with cable lengths between the power supply and the load of 100 feet or more during and after load arcing.